Targets for x ray tubes

ABSTRACT

An X-ray tube including a target structure having a focal area for receiving electrons from an adjacent cathode and generating X-radiation in response thereto, the focal area being provided with a surface designed to improve thermal properties such as by covering with a closely packed wrapping of relatively fine wire, the focal area preferably being a wire-wrapped annulus sandwiched between two discs of high thermal conductivity material.

United States Patent 1191 Holland et al.

1111 3,821,581 June 28, 1974 TARGETS FOR X-RAY TUBES [75] Inventors: William P. Holland, West Redding;

Thomas J. Koller, Trumbull, both of Conn.

[73] Assignee: The Machlett Laboratories,

qrpq. 1 s priq a 19 m [22] Filed: Aug. 2, 1971 y [2]] Appl. No.: 168,330

947,998 8/1956 Germany 313/330 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm-Harold 'A. Murphy; Joseph D. Pannone; John T. Meaney [57] ABSTRACT An X-ray tube including a target structure having a focal area for receiving electrons from an adjacent cathode and generating X-radiation in response thereto, the focal area being provided with a surface designed to improve thermal properties such as by covering with a closely packed wrapping of relatively fine wire, the focal area preferably being a wirewrapped annulus sandwiched between two discs of high thermal conductivity material.

8 Claims, 9 Drawing Figures mmmmw TARGETS FOR X-RAY TUBES 7 BACKGROUND OF THE INVENTION In the manufacture of targets for X-ray tubes, the

.portion of the target which is to be subjected to elecseriously damaged through high thermal gradients causing severe mechanical stresses which result from bombardment by high energy electrons. This produces cracking, warping, and focal area disruption. For example, the temperature assumed by a conventional tungsten target at the focal spot may approach 3,400C and such heat may create hoop stresses which produce cracking, resulting in mechanical failure, or warpage which alters the target angle and thereby changes the focal spot size.

' Attempts to overcome these and other problems have been made by forming a target of a selected refractory base material having high thermal capacity, such as molybdenum or graphite, for example. On this base material is deposited a layer of high atomic number material which has high melting point and low vapor pressure. This layer, which may be vapor deposited, flame sprayed, or brazed, may cover one side of the base or may cover the entire base surface. Such materials as rhenium, tungsten, or suitable alloys are deposited in the selected area or areas and are attached by a metallurgical bond to the base material.

These coated targets, however, have also been found to be unsatisfactory because of the extreme difficulty in obtaining good adhesion of the deposit to the base material. Differences in thermal expansion coefficients have caused much of the failure in devices of this character.

SUMMARY OF THE INVENTION The above and other objections to the prior art are overcome in the present invention by a novel target structure which comprises a focal area which is provided with a surface of increased area for improved thermal properties, such being achieved by locating on the surface a layer of relatively fine closely packed wires. This may be done, for example, by wrapping the entire focal area in a wire winding.

In a rotating anode X-ray tube the target comprises a disc having an inclined annular focal track defining a portion of a cone. This focal track is covered by wires disposed in close side-by-side relation and extending along the generatrices of the cone in accordance with this invention. By generatrices is meant lines extending radially from the tip of the geometric cone to points on the outer periphery of the cone.

The annular focal track area may comprise a portion of a one-piece solid disc or, preferably will comprise a separate annulus or ring having the wire wrapped around it, the ring thereafter being sandwiched between two discs of high thermal conductivity material. The focal area may be any suitable refractory material such as tungsten or tungsten-rhenium alloy, for example. The supporting discs may be any refractory material such as molybdenum, titanium or graphite, for example. In a fixed anode tube, the target may comprise a wire-wrapped disc inset into a block of copper or the like.

More specifically, the rotating anode may be basically constructed as shown and described in US. patent application Ser. No. 230,053, filed Feb. 28, I972.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other advantages of the invention will become apparent from the following description taken in connection with the accompanying drawings, wherein 1 FIG. 1 is an axial section through an X-ray tube of the rotating anode type showing a target structured in ac cordance with this invention;

FIG. 2 is an elevational view of the target in the tube of FIG. 1;

FIG. 3 is an axial section through the target of FIG. 2;

FIG. 4 is an enlarged view of a portion of the target of FIG. 3;

FIG. 5 is ,an enlarged elevational view of a portion of a target focal track;

FIG. 6 is a diagram illustrating an electron beam impinging upon a target;

FIG. 7 is a fragmentary elevational view ofa modified rotating anode target;

FIG. 8 is a fragmentary axial sectional view of the target shown in FIG. 7, and

FIG. 9 is a fragmentary axial sectional view of a portion of a stationary anode X-ray tube having a target embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to the drawings, there is shown in FIG. 1 an axial sectional viewof an X-ray tube of the rotating anode type which embodies a dielectric envelope 10 in which is supported an anode l2 and a cathode 14. The cathode 14 includes a supporting cylinder 16 one end of which is sealed to a reentrant end portion 18 of the envelope. On the inner end of cylinder 16 is mounted one end of a transversely extending angled support bracket 20, in the free end of which is located a cathode head 22. The cathode head 22 contains an electron-emitting filament (see FIG. 3) to which a suitable electrical potential is applied through leads 24 extending externally of the tube through cylinder 16.

The opposite end of the envelope 10 carries the anode 12 which includes a target 26 mounted on one end of a rotor shaft 28 extending from a rotor 30 rotatably located in a neck portion 32 of the envelope. The rotor carries a skirt 34 bolted thereto, and the assembly is adaptedto rotate rapidly when the tube is mounted in suitable inductive means surrounding the neck 32 when the inductive means is energized.

In accordance with this invention, the anode target assembly comprises a focal track member 36 in the form' of a ring made of suitable high atomic number material, such refractory materials as tungsten or tungsten-rhenium being particularly suitable. The focal track member 36 produces X-rays when bombarded by electrons from the cathode 22 in the usual manner of X-ray generators. The exposed surface or track of the focal target member 36 is inclined so that X-rays will pass from the surface out of the tube through the side wall of the envelope. This surface defines a portion of a cone having its longitudinal axis coincident with the axis of the target 26 and particularly of the rotor shaft 28.

The target 26 is preferably made of a plurality of superimposed members as described in the aforementioned US. application Ser. No. 230,053. Briefly, the focal track member comprises a ring 36 having its lower side positioned upon a surface of a backing disc 38, which surface is shaped to receive the adjacent surface of the focal track ring. For this purpose the disc 38 is provided with a peripheral groove 40 in its upper surface within which the ring is nested. The upper surface of the focal track ring 36 is provided with a circumferential ridge 42 which inclines on either side thereof to form first and second or inner and outer inclined surfaces 44 and 46 respectively. Outer surface 44 is the exposed focal track surface and is at an appropriate predetermined angle to provide proper focal spot size, as will be described.

The inner surface 46 is inclined so as to effectively produce an annular recess within which a second backing ring or dome 48 is nested in a manner which permits thermal expansion and contraction between the parts without damage. With a nested structure of this type the focal track ring 36 is firmly engaged by adjacent surfaces of the backing disc 38 and dome 48 throughout relatively expansive surface areas to achieve efficient conduction of heat from disc or ring 36 to the disc 38 and the dome 48 as is desired.

When mounting a target assembly 26 on its supporting anode structure, it is important to insure that the two rings or discs 36 and 38 are at'all times held in the required closely abutting relationship. Therefore, there must be some means provided for this purpose. In the example shown in FIG. 1, this is achieved by providing the rotor shaft 28 with a collar 50, and backing ring 38 is mounted over the end of shaft and seated against a ring 52 which rests upon the collar. The lower surface of the backing ring may be suitably recessed as illustrated to receive the ring 52. Then the focal track ring 36 is slid downover the shaft into intimate engagement with the backing ring 38. As shown in FIG. 1, the second backing ring or dome 48 is then mounted on the shaft 28 and slid down into intimate physical contact with the adjacent inclined surface 46 of ring 36, and the complete assembly is compactly and firmly pressed into an assembled unit by means such as a nut 52 which is threaded onto the end of the shaft into engagement with a cup-shaped retainer 54 carried by the second backing ring or dome 48, as illustrated.

From the above it will be apparent that when the tube is operated a stream or beam of electrons will be emitted by the cathode 22 in the well-known manner and will impinge upon the adjacent inclined surface 46 of the focal track 36, whereupon x-radiation will be emitted by this surface and will pass out of the tube through the X-ray' transparent wall of the envelope 10. During this operation considerable heat is generated within the target ring 36. Therefore, to partially aid in the distribution of heat throughout the ring, as opposed to a localized area thereof, the target assembly 26 is caused to rotate at a relatively high speed so that a new surface area is constantly and continuously being presented to the electron beam, as is well known.

It is known, however, that X-ray tube anodes have a limited high load capability determined by the electron beam current density, among other parameters. At sufficiently high current densities even rotating anodes have been found to produce some surface melting and this, therefore, requires that limits be set on X-ray pulse duration and intensity of tube loading. Heat dissipation in X-ray anodes is accomplished primarily through radiation.

It is well known that improvements can be achieved by selection of materials for the focal track area of a target which have desirable thermal dissipation properties. Graphite is a particularly suitable material for this purpose while tungsten, rhenium-tungsten alloy, and molybdenum are examples of other relatively satisfactory materials from which the focal track ring 36 may be made. Graphite, tungsten or molybdenum, for example, may be employed as the material for the base ring or disc 38 and the dome 48.

However, even greater performance may be achieved by increasing the surface area of the focal track ring 36 in accordance with this inventionjA preferred structure of this type is depicted in FIGS. 2-6 wherein the focal track ring 36 is fabricated with a closely packed wrapping of wire formed of suitable target material such as tungsten, rhenium-tungsten alloy or molybdenum, for example. The wire 60 is tightly wrapped throughout the annulus of the ring 36 so that the individual turns extend substantially radially from the geometric center of the ring.

The wire 60 is preferably of a diameter selected in accordance with the size of the focal spot which is to be produced by the tube. If the focal spot is to be relatively large, for example 1.5-2 mm. wide, the wire will preferably be about 0.008 inch in diameter. If a small focal spot of 0.3-1 mm., for example, is to be produced, the wire diameter will preferably be about 0.0035 inch. These examples of wire. size are only exemplary and the wire diameter may vary substantially from these examples.

While tungsten, rheniumdungsten alloy, and molybdenum have been mentioned above asthe material for the wire, the relative simplicity of the wire-making process, in comparison to known processes for making X-ray targets, permits a much wider choice of materials for this purpose. Additionally, during a wiremaking process it is possible to add selected doping materials or other additives which may control X-ray generating or other characteristics of the resultant wire.

The term focal spot refers to the spot or area of the target surface which is impinged by electrons from the cathode 22. Referring to FIGS. 3 and 5, the length of the focal spot 62 will becontrolled primarily by the length of the filament in the cathode and somewhat by the geometry of thefocusing cup or grid (not shown) in which the filament is mounted, as is well known. The width dimension B of focal spot 62 is almost entirely controlled by the grid or cup as is also well known.

Therefore, when a tube is operated and a cathode 22 is pulsed to emit electrons in a normal manner, the electrons will bombard the target in the area indicated by numeral 62 in FIG. 5 However, the resultant X-ray beam to be utilized will be directed alonga path extending from the spot 62 in a direction substantially perpendicular tothe axis of the tube. This effective X-ray beam is directed, therefore, in the general direction indicated by the dotted lines denoted C in FIG. 3, which also indicate the width of the focal spot as viewed along a line perpendicular to the tubes longitubeam from cathode 22 will impinge upon any number of turns of wire 60, depending upon the size of the wire and of the filament, as well as the electrical effect of the cathode upon the beam. This is illustrated diagrammatically in FIG. 6 wherein lines 66 denote an electron beam which impinges upon wires 60 carried by focal track ring 36.

It will be seen that the beam 66 impinges simultaneously upon a number of turns of wire 60. Therefore,

electrons will fall not only on the surfaces of the wires which lie closest to the cathode but will also fall upon some of the side surfaces of the wires. This, consequently, effectively distributes the electrons over a greater surface area of the target than is possible when the focal track surface is relatively smooth. As a result, some of the electrons penetrate less deeply into the bombarded surface. This all leads to the generation of less heat in the same surface of the target than was pre-. viously the case by more widely distributing such heat, permitting tubes to be operated at higher power levels without target damage.

It will be understood, however, that the turns of the wire 60 should be relatively tight. Therefore, the under surface of the focal track ring 36 is provided with an annular circumferential groove 70 as shown in FIG. 4, and the adjacent surface of supporting disc on ring 38 is provided with an annular projection or ridge 72 which is aligned with the groove 70 and extends thereinto as shown. When disc 38 is being assembled with ring 36 the ridge 72 projects into the aligned groove 70 and in doing so it exerts considerable pressure upon the adjacent portions of the wire 60, tending to tighten it.

It will be understood that possibly with some wire sizes some of the turns of wire 60 may be slightly spaced from adjacent turns. This is the case near the outer peripheral edge of surface 46 when extremely fine wire is being employed. However, in such an event the electrons impinging upon the target in these areas will engage the small exposed surface areas of the focal track surface 46 and will generate X-radiation directly from this surface. Alternatively, the layer of wire may be overlaid with an additional layer or layers of windings to cover the exposed surface areas of the focal track surface.

It will be noted that the convolutions provided by the wire should extend radically from the geometric center of the focal track ring 36. If the convolutions were to extend perpendicularly in the direction of the curve of the annulus, it will be apparent that some of the high portions of the convolutions will effectively intercept and block escape of some of the desirable X-radiation.

In some cases it would be satisfactory to form the convolutions by providing radically extending grooves 80 (FIGS. 7 and 8) in the exposed surface of a focal track ring 82. In this modification the target 84 is constructed as shown and described in aforementioned application Ser. No. 230,053. Grooves of this type are disclosed in a solid one-piece target in the US. Pat. No. 2,071,696 which does not, however, provide the improved heat dissipation characteristics achieved by the combination with the multi-piece target.

It is to be further understood that a stationary anode X-ray tube may be provided with this invention as illustrated in FIG. 9. p

While the foregoing description relates primarily to X-ray tubes having rotating anodes, the invention is also particularly well suited for use in stationary anode tubes such as, for example, the type shown in FIG. 9. The stationary anode tube shown in FIG. 9 includes an envelope within one end portion of which is an anode 88. Anode 88 is a body of copper, usually, which is provided with a hollow cylindrical extension portion 90 having an open end directed toward the cathode. An X-ray emitting target button 92 is provided in the base of the cavity thus formed in the anode for the purpose of receiving electrons from the cathode and directing resultant X-rays out through an opening 94 and then through the wall of the envelope 68.

In accordance with the present invention, a block or body of graphite, copper, or other selected backing material 96 is deposited in the bottom of the hollow anode extension and is provided with an inclined surface having a recess therein in which the target button 92 is positioned. A sleeve or shell 98 of graphite or other selected high thermal capacity material is then positioned in the extensionwith one end thereof engaging the target button 92. Sleeve 98 is provided with an opening 97 which may contain a window 99 of beryllium or other material highly transmissive to X-radiation which is suitably aligned with opening 94 in extension 90 whereby X-rays emanating from the target button 92 will pass outwardly through the window 99 and opening 94.

In accordance with the present invention the target button 92 is covered with a wire winding 100 similar to the winding in the rotating anode described hereinbefore. In the stationary anode, however, it is important that the wires extend substantially in the direction of the angle of inclination of the button 92 so that the convolutions formed by the wires will not block escape of any of the X-radiation out through window 99. Thus, there is provided a stationary anode target which has the improved capabilities described in connection with the rotating anode.

In accordance with the foregoing description it will be understood that a focal track surface area is increased 1r/2 that offlat surfaces whereby greater power loads can be applied to the focal track. In addition, the

increased surface allows greater radiative heat dissipasense.

We claim: 1. A target for X-ray tube anodes comprising a target member of material capable of X-rayemission when a surface thereof is impinged by electrons, and at least one backing member of high thermal capacity material engaging one side of the target member, said target member being an annulus extending around a perpen- 7 dicular axis, and said surface of the target member is provided with means for increasing the area thereof, said means comprising at least one layer of wires arranged in side-by-side relation, each individual wire extending substantially radially with respect to said axis.

vfitting means comprises an annular ridge in the adjacent abutting surface of the backing member and a comating annular groove in the target member, and further comprises means for mechanically clamping said members together to force the ridge into the groove and thereby tightening the wire therebetween.

5. An X-ray tube comprising a hermetically sealed envelope, a cathode electrode and an anode electrode located in spaced relation within the envelope, and means for connecting said electrodes to external sources of electrical energy, said anode comprising an axial support, and an X-ray generating target assembly mounted on the support, for rotation about the axis of the support said target assembly comprising a first member composed primarily of high thermal capacity material sandwiched between second and third members of high thermal capacity material, a focal track area of said first member being exposed, and means for mechanically retaining said members in intimate physical heat conductive relation, said focal track area comprising parallel corrugations, said corrugations comprising surfaces of wires disposed on said area in substantially side-by-side relation, each individual wire extending substantially radially of said axis.

6. An X-ray tube as set forth in claim 5 wherein said first member is covered by a wire winding and said corrugations comprise a surface of said winding.

7. An X-ray tube as set forth in claim 6 wherein said first member is an annulus extending about a geometric axis, and the turns of said winding extend substantially radially with respect to said center.

8. An X-ray tube as set forth in claim 7 wherein abutting surfaces of said first member and one of'said other members are provided with interfitting means which v engages and stretches said winding therebetween. 

1. A target for X-ray tube anodes comprising a target member of material capable of X-ray emission when a surface thereof is impinged by electrons, and at least one backing member of high thermal capacity material engaging one side of the target member, said target member being an annulus extending around a perpendicular axis, and said surface of the target member is provided with means for increasing the area thereof, said means comprising at least one layer of wires arranged in side-by-side relation, each individual wire extending substantially radially with respect to said axis.
 2. A target as set forth in claim 1 wherein said wires comprise parts of turns of a wire winding wound around the annulus.
 3. A target as set forth in claim 2 wherein said target member and an adjacent abutting surface of the backing member are provided with interfitting means for tightening said wire winding.
 4. A target as set forth in claim 3 wherein said interfitting means comprises an annular ridge in the adjacent abutting surface of the backing member and a comating annular groove in the target member, and further comprises means for mechanically clamping said members together to force the ridge into the groove and thereby tightening the wire therebetween.
 5. An X-ray tube comprising a hermetically sealed envelope, a cathode electrode and an anode electrode located in spaced relation within the envelope, and means for connecting said electrodes to external sources of electrical energy, said anode comprising an axial support, and an X-ray generating target assembly mounted on the support, for rotation about the axis of the support said target assembly comprising a first member composed primarily of high thermal capacity material sandwiched between second and third members of high thermal capacity material, a focal track area of said first member being exposed, and means for mechanically retaining said members in intimate physical heat conductive relation, said focal track area comprising parallel corrugations, said corrugations comprising surfaces of wires disposed on said area in substantially side-by-side relation, each individual wire extending substantially radially of said axis.
 6. An X-ray tube as set forth in claim 5 wherein said first member is covered by a wire winding and said corrugations comprise a surface of said winding.
 7. An X-ray tube as set forth in claim 6 wherein said first member is an annulus extending about a geometric axis, and the turns of said winding extend substantially radially with respect to said center.
 8. An X-ray tube as set forth in claim 7 wherein abutting surfaces of said first member and one of said other members are provided with interfitting means which engages and stretches said winding therebetween. 